5 Answers
5

The aperture range on your lens only shows the maximum aperture for your lens at the extremes of the zoom range; i.e. f/3.5 at 18mm and f/5.6 at 55mm. There is nothing to stop you using a narrower aperture; remember a larger number is a smaller hole (the f number is the diameter of the hole as a fraction of the focal length).

The narrowest aperture is often something like f/22 (or f/16, or similar) and does not change throughout the zoom range.
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thomasrutterMar 21 '11 at 5:16

5

@thomasrutter I don't think that the second part is correct. The narrowest aperture does change, at least on the Canon equivalent.
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damned truthsFeb 8 '13 at 6:08

I don't know what I was thinking when I wrote that comment but you are right, often the narrowest aperture does change over the zoom range. However, it's somewhat irrelevant because you would rarely want to use the narrowest aperture due to diffraction.
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thomasrutterFeb 10 '13 at 5:46

When you buy a lens, the numbers on it describe the range of zoom, and the maximum aperture. If the aperture is listed as a range, it is the corresponding maximum aperture at the lowest and highest zooms. Minimum apertures are not usually described in a lens' name. Thankfully, my f/2.8 lenses can stop down lower, or I'd have some real depth-of-field issues.

If your maximum aperture is 3.5 at 18mm, then the leaves on the aperture are fully open, and that's the maximum amount of light permitted. When you adjust it to be 55mm, and the aperture is 5.6, it is not the leaves that are the limiting factor, it is the construction of the lens body. The shutter leaves, even if fully opened to what would be the 3.5 position, still cannot let in more light than a 5.6 aperture because the shutter leaves are no longer the greatest constriction when your lens is zoomed.

Therefore, setting an aperture of 5.6 will always result in a 5.6 on that lens, and the same goes for any higher setting as the aperture leaves are the greatest point of constriction in the path. The path of constriction would be roughly related to the cone formed between the front element and the rear element as related to the field of view (focal length). On narrower (and cheaper) lenses, this constriction is greater.

I think this question may benefit from an answer that goes a little further back to basics than the answers I see so far.

The word aperture literally just means opening. "An aperture" is a phrase that could be used to describe any opening. In the world of photography, though, the word aperture is usually used to refer to a very specific opening inside a camera's lens. What is it an opening in? Well, inside the lens, there are a set of metal "blades", whic overlap each other. The opening, or aperture, can be opened and closed (in a roughly circular configuration - though it may actually be a pentagon or an octagon or other shapes, depending on the construction of the specific lens in question). The way photographers indicate how open or closed the aperture is is with an f-number. Examples might be f/3.5, f/5.6, f/9, or f/11. You may notice I write these all as "eff over" something. The "over" is often left out of speech (one might just say "eff five point six"), but it is quite relevant. The number is actually an expression of the ratio between the focal length of the lens ("f") and the diameter of the opening - so, for example, f/4 on a 100mm lens means that the diameter of the aperture's opening (possibly redundant phrase, but in this case I mean the mechanical blades as the "aperture", which thus does have an opening) is 25mm. At f/8, the opening has a diameter of 12.5mm, etc. This also means that the same f-number on a different focal length lens will have a different opening diameter - while f/8 was 12.5mm for a 100mm lens, that same diameter will be f/4 on a 50mm lens, or f/2 on a 25mm lens.

So, with a zoom lens, you get an interesting thing going on: if you leave the aperture blades in their same physical position, and zoom in or out, this has the effect of changing the f-number, without changing the actual aperture diameter. With many zoom lenses, including yours, the default way of operating is to do this when your aperture is set to be all the way open. Some zoom lenses, especially "fast" and/or high-end ones, have a "constant" maximum (or "wide open", i.e. the all-the-way-open setting of the aperture blades) aperture, which actually does change the aperture diameter as you zoom, even when wide open (even your lens, when set to a non-wide-open aperture like f/8, will change its aperture diameter).

So, on your lens, the listing of f/3.5-f/5.6 is an indication of how wide the aperture is when all the way open. The aperture can still close down from that point, though. You may also find that the minimum (smallest opening, though that's the highest number) aperture also changes as you zoom - perhaps f/22 at the wide end of its range, and f/32 on the telephoto side of things. This would be an indication of the same sort of thing as on the other end, just in reverse - the blades are only able to close to a certain physical diameter, but that diameter yields a different f-number depending on the focal length of the lens.

I hope this all makes sense - I wrote it in small chunks over several hours from my mobile device. If anyone has any questions, I'll try to re-visit it and clean things up. :)

Edit (much later): I think there's actually more going on here than I was aware of when I wrote this answer (in particular: apparent aperture size as well as actual aperture size). I still don't fully understand the details, so I won't try to "correct" this answer (I think it's still mostly a reasonable answer to this question, as asked), I'll merely point out that this may be less than complete and/or, potentially, have portions that are misleading and/or false. (If so, I apologize!) More information is also available in (answers to) another question, so, readers are encouraged to look there, as well.

Take a manual lens that is similar... my memory tells me I used to have a, 28-80 3.5-5.6 Pentax mount. The aperture blade position was set with the aperture ring, and that was independent of the zoom. As it was fully manual, the camera had a lever which pressed against an arm sticking out of the lens -- blades open, or blades closed. Thus, an f/22 on that lens resulted in the same physical aperture opening at any focal length. While I observe that my fully auto cameras do have differing min apertures based on zoom, that conflicts with what I know from the fully manual. Can you reconcile that?
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Jeff FerlandMar 20 '11 at 4:25

It depends on where in the optical path the adjustable aperture is (where the leaves are). On some lenses they are nearer the entrance, and on others they are nearer the node point (the narrow end of the focus cone). The Pentax Auto 110 (a 110 SLR) had the aperture in the body, and all lenses were f/2.8 wide open regardless of the focal length. Since the "choke point" was at the node, the same f-stop represented the same physical aperture size on all lenses, so diffraction was as problematic at f/8 on their longest tele as it was on their shortest wide angle.
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user2719Mar 20 '11 at 22:30

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With constant aperture zoom lenses, the actual aperture doesn't necessarily change as you zoom, the apparent aperture does. The apparent aperture is the size of the aperture as measured by observing through the lens' objective and will be affected by the magnification factor and distances of lens elements between the front of the lens and the aperture blades.
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Michael ClarkFeb 7 '13 at 6:39

Thanks, @MichaelClark! I've added an edit with a link to another question that goes into more detail about that case.
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lindesFeb 7 '13 at 13:02

How can that be, that f-number is greater than 5.6, when that's listed as the maximum in the lens spec?

Because you've misinterpreted the lens spec. When it says f/3.5 - 5.6, it means the widest aperture (wider = lower f-number) is f/3.5 at one end of the zoom range and f/5.6 at the other. The narrowest aperture is not usually quoted, but is usually something like f/22 or thereabouts - sometimes even narrower.

I feel this is technically correct and yet fails to answer the question being asked.
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Jeff FerlandMar 20 '11 at 4:26

I have to agree with @JeffFerland. You've explained why f/11 is smaller than f/3.5, but have not explained why f/11 is possible at all on an "f/3.5 lens", which I believe is what the question is asking.
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jrista♦Mar 22 '11 at 5:17

My feeling was that the source of the asker's confusion was the backwards-appearing nature of the f-stop scale. Once you see how it works, one won't wonder anymore how the f-number can be greater than the maximum, because it becomes obvious that it isn't. If explaining it this concisely is no help, then I guess my answer will languish with zero points. Oh well.
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Lyman Enders KnowlesMar 22 '11 at 23:31